The invention relates to a method as defined in the preamble of claim 1 for feeding solids into a reaction space, the solids being consumed during the reaction that takes place in the reaction space.
In processes where solid matters are transformed into non-solid matters during the reaction, the methods for feeding solid matters are invariably based on the use of sensors for measuring various variables. The sensors are used for measuring either process parameters or any variable consumed in the process, and the feed of solid matters into the reaction is controlled on the basis of the signals emitted by the sensor. Such variables to be measured usually comprise for instance the solids content or product content, the process temperature or the surface level or weight of the solids.
However, control methods based on sensors involve a number of drawbacks. Control methods based on the use of sensors always involve a certain delay, which depends on the process and the sensor in question and causes fluctuation in the process control. Thus, for instance, the delay of oxygen sensors for observing flue gases is in the range from tens of seconds to about one minute, and then there will continually be a small lag between the process steps and the process control. The practical inconvenience caused by this fluctuation depends on the nature of the process to be controlled. In chemical processes, the use of sensors tends to create problems in the dissolution processes of solid matters, i.a. salt, and the combustion processes of solid fuels. In such processes, even small lags between the solids feed rate and the reaction rate of the solid matter consumed in the process result in fluctuating process parameters, such as for instance the solution concentration, the process efficiency or emissions. Specifically in the case of the combustion process of solid fuels, the entire process is apt to fluctuate, because fuels react in different ways at various stages of the heating of the fuel. The problem is particularly accentuated in the gasification of solid fuels in a solid bed process, because fuel consumption is impossible to measure in practice by any other means than sensors based on the physical shape of the fuel layer. Since solid fuels retain their shape for some time regardless of the reactions of the substance, the use of sensors based on the physical shape, such as the surface, tends to entail clear lags between the fuel feed rate and the reaction rate, resulting in fluctuations in the entire gasification process. When gas is used as an engine fuel, this may in the extreme lead to the engine stalling if the heat content of the gas varies too much.
The object of the invention is to eliminate the drawbacks of conventional methods, in which solid matters are fed into a reactor, where these solids are transformed into non-solid substances during the reaction. In such processes, the lags between the feed rate and the reaction rate of solids impair the outcome of the process in most cases, and for this reason the chief objective of the invention is to eliminate fluctuation by providing a method for feeding solid matters, in which the solids feed rate is permanently essentially equal to the solids reaction rate in the reactor.
The objective mentioned above is achieved with the method of claim 1 and the apparatus of claim 7 used in this method.
In the method of the invention, solids are fed into a reaction space, which the solids are prevented from leaving in the form of solid substances. Solids are fed into the reaction space under constant feed pressure, i.e. with constant torque, and then the feed will exert constant pressure on the walls of the reaction space or the walls of the feed duct leading to the reaction space. At the same time, the reaction space is supplied with a substance reacting with the solids and resulting in the conversion of the solids into a non-solid reaction product. During the reaction, the counter-pressure exerted by the walls of the reaction space or the walls of the feed duct leading to the reaction space on the solids decreases below the feed pressure due to the consumption of solids, and consequently, solids are impelled into the reaction space at the same rate as they are being consumed during the reaction. On the one hand, the apparatus of the invention comprises means for feeding solids and also the substance reacting with the solids into the reaction space, and the reaction space comprises means for preventing the solids from leaving the space as solid substances. The feed apparatus, on the other hand, comprises a feed device and means for maintaining the feed pressure of the feed device constant.
The method of the invention and the apparatus used in it allow fluctuation to be eliminated in processes where solid matters are fed into the reactor, where the solids are converted into non-solid reaction products. This is achieved with the method for feeding solids of the invention, where solids are fed into the reaction space consistently at the same rate as they are being consumed in the reaction. The feed method is based on the fact that, as the solids react and are thus consumed, the counter-pressure exerted by the walls of the reaction space or the walls of the feed duct on the solids drops, the permanently constant feed pressure thus exceeding the counter-pressure, so that solids are automatically impelled into the reaction space. Having a constant torque, the feed device thus keeps the constant amount of reacting solids in the reaction space. Whether a high or a low reaction rate, the contact with the solids and the substances reacting with them will remain constant, and no fluctuation in the lag between the solids feed rate and the reaction rate, which would impair the reaction efficiency, will occur.
The solids feed is preferably performed by means of a hydraulic device utilising the pressure of a hydraulic liquid, such as a hydraulic motor or a hydraulic cylinder. The method is particularly well applicable to the control of a gasification process, in which the solid substance to be burnt is a solid fuel, such as for instance biomass, and the substance reacting with it is air or oxygen.